1. Field of the Invention
The present invention relates to an IBAD apparatus and an IBAD method, both of which are used for the manufacturing of a base material for oxide superconducting conductors.
2. Description of the Related Art
Since an RE-123-based oxide superconducting conductor (REBa2Cu3O7-x: RE is any one of rare-earth elements including Y) exhibits excellent superconductivity at liquid nitrogen temperature or more, it is regarded as a very promising material for practical use and it is strongly desired to use the RE-123-based oxide superconducting conductor as a conductor for electric power supply by processing it into a wire. As a conductor which is used for such an RE-123-based oxide superconducting conductor, as shown in FIG. 7, a structure formed by laminating an intermediate layer 102 formed by an IBAD (Ion Beam Assisted Deposition) method, a cap layer 103, and an oxide superconducting layer 104 in this order on a tape-shaped metal base material 101 is known (refer to the following Japanese Unexamined Patent Application, First Publication No. 2004-71359, for example).
In the oxide superconducting conductor, the intermediate layer 102 and the cap layer 103 are provided to control the crystalline orientation of the oxide superconducting layer 104. That is, an oxide superconductor has electric anisotropy in which electricity easily flows in an a-axis direction and a b-axis direction of the crystal axes thereof, but electricity does not easily flow in a c-axis direction. Therefore, in a case where a conductor is constituted using the oxide superconductor, the oxide superconducting layer 104 requires that an a-axis or a b-axis be oriented in the direction of flowing electricity and a c-axis be oriented in the other direction.
Here, as a technique of forming the intermediate layer 102 which is used for this type of oxide superconducting conductor, the IBAD method is widely known. An intermediate layer which is formed by the IBAD method is constituted by a material in which a physical characteristic value such as coefficient of thermal expansion or lattice constant shows an intermediate value between the metal base material 101 and the oxide superconducting layer 104, for example, MgO, YSZ (yttria-stabilized zirconium), SrTiO3, or the like. Such an intermediate layer 102 serves as a buffer layer which reduces a difference in physical characteristics between the metal base material 101 and the oxide superconducting layer 104. Further, by forming the intermediate layer 102 by the IBAD method, the crystal of the intermediate layer 102 has a high degree of in-plane orientation and the intermediate layer 102 serves as an orientation control film which controls the orientation of the cap layer 103. An orientation mechanism of the intermediate layer 102 which is formed by the IBAD method will be described below.
As shown in FIG. 8, an apparatus for forming an intermediate layer by the IBAD method has a traveling system for making the metal base material 101 travel in the longitudinal direction thereof, a target 201, the surface of which obliquely confronts the surface of the metal base material 101, a sputter beam irradiation device 202 which irradiates the target 201 with ions, and an ion source 203 which irradiates the surface of the metal base material 101 with ions (mixed ions of rare gas ions and oxygen ions) from an oblique direction. These respective sections are disposed in a vacuum container (not shown).
In order to form the intermediate layer 102 on the metal base material 101 by this intermediate layer forming apparatus, the inside of the vacuum container is set to be in a reduced-pressure atmosphere and the sputter beam irradiation device 202 and the ion source 203 are then operated. In this way, ions are irradiated from the sputter beam irradiation device 202 to the target 201, so that constituent particles of the target 201 are splashed and deposited on the metal base material 101. At the same time, the mixed ions of rare gas ions and oxygen ions are radiated from the ion source 203, thereby being incident on the surface of the metal base material 101 at a given incidence angle (θ).
In this manner, by performing ion irradiation at a given incidence angle while depositing the constituent particles of the target 201 on the surface of the metal base material 101, a specific crystal axis of a sputtered film being formed is fixed in the incident direction of the ions. As a result, the c-axis is oriented in the vertical direction with respect to the surface of the metal base material and also the a-axis and the b-axis are oriented in given directions in a plane. For this reason, the intermediate layer 102 formed by the IBAD method has a high degree of in-plane orientation.
On the other hand, the cap layer 103 is constituted by a material, for example, CeO2, which epitaxially grows by being formed on the surface of the intermediate layer 102 with the in-plane crystal axes oriented in this manner, and thereafter, performs grain-growth in a lateral direction, so that crystal grains can be self-oriented in an in-plane direction. The cap layer 103 is self-oriented in this manner, thereby being able to obtain a higher degree of in-plane orientation than that of the intermediate layer 102. Therefore, if the oxide superconducting layer 104 is formed on the metal base material 101 with the intermediate layer 102 and the cap layer 103 interposed therebetween, the oxide superconducting layer 104 epitaxially grows so as to conform to the crystalline orientation of the cap layer 103 having a high degree of in-plane orientation. For this reason, it is possible to obtain the oxide superconducting layer 104 having excellent in-plane orientation and large critical current density.
FIG. 9 shows a schematic structure example of a specific apparatus in the case of carrying out the above-described IBAD method. The configuration of an IBAD apparatus 300 of this example will be described below. A long tape-shaped base material 301 is wound a plurality of times round a first roll 302 and a second roll 303 to travel back and forth. A rectangular target 305 is disposed so as to face the base material 301 which is exposed in a plurality of rows between the first roll 302 and the second roll 303. A sputter ion source origin 306 is disposed so as to face the target 305 in an oblique direction. An assist ion source origin 307 is disposed so as to face the base material 301 which is exposed in a plurality of rows between the first roll 302 and the second roll 303, from an oblique direction with a given angle (for example, 45° or 55° with respect to a normal of the film formation surface of the base material 301).
In addition, as another ion beam sputtering apparatus, as described in the following Japanese Unexamined Patent Application, First Publication No. 2004-027306, an apparatus is known which adopts a configuration in which a plurality of ion guns is provided so as to correspond to a plurality of targets, and has a configuration in which two sets of ion guns are disposed at the symmetric positions of a rotating holder provided with the targets. Further, an apparatus having a configuration in which a plurality of ion guns is provided is also known. Further, as described in the following Japanese Unexamined Patent Application, First Publication No. 8-74052, an ion beam sputtering apparatus is known in which a plurality of ion guns is provided with respect to a single target. Further, as described in the following Japanese Unexamined Patent Application, First Publication No. 2004-285424, an ion beam sputtering apparatus is known in which a plurality of ion gun drives is provided to irradiate a plurality of areas of a target with ion beams and current density distribution is controlled for each ion beam irradiation position.
The IBAD apparatus 300 shown in FIG. 9 is advantageous when trying to form a thick film and has a feature that enables productivity to be improved, because the base material 301 travels back and forth a plurality of times between the first roll 302 and the second roll 303. However, in the IBAD apparatus 300 shown in FIG. 9, in order to make sputter particles uniformly reach a plurality of rows of base materials 301 which are spanned across the first roll 302 and the second roll 303, the sputtering target 305 is made to have a large rectangular shape. In response to this, the sputter ion source origin 306 is also made to have a rectangular shape. Since such a large rectangular ion source origin 306 is not commonly used in a general film-formation field such as a semiconductor field, but a special order product, it has the problem that it is inevitably very expensive.
Further, in the large IBAD apparatus 300 as shown in FIG. 9, in order to maintain uniformity of a film thickness and film quality, it is necessary to balance the intensities of the assist ion beam and the sputter ion beam. In this type of ion beam sputtering apparatus, usually, one set of assist ion gun and one set of the sputter ion gun are present. However, in a case where an ion source of a sputter beam is one, it is extremely difficult to further adjust a film thickness while forming a film on a large area.
The present invention has an object to provide an IBAD apparatus and an IBAD method, both of which are suitable for the manufacturing of a base material for an oxide superconducting conductor having an intermediate layer having excellent crystalline orientation and a uniform film thickness, which is a base material having an intermediate layer that becomes a basis for forming an oxide superconducting layer having excellent crystalline orientation.